First, go to Twinblade's camp and get as many bandits as possible to follow you. Lead them through Darkwood to the Greatwood Caves where the Demon Door is found. Slow Time and Assassin Rush are needed in Darkwood so they do not die. Once you get to Greatwood Caves, Slow Time and kill the Earth Troll or the Hobbes (depending on if you have done the Rose Cottage side quest). You must get a combat multiplier of +14 for the Demon Door to open. When you have killed either the Hobbes or Earth troll, start attacking the bandits. As soon as your combat multiplier is high enough, run over to the door and talk to it. After going through the door, you will find the Cutlass Bluetane, which has a lightning augmentation and does 165 damage.